In paper coating, a coating layer is applied onto a base paper web manufactured on a paper machine, and then the coating is doctored to a target thickness. The coating is a mixture of water and solids, and accordingly, the web must be dried before it can be taken to subsequent treatment steps. Various kinds of dryers are known and they can be divided into, e.g., non-contact dryers and contact dryers. Dryers which touch the web comprise different kinds of heated cylinders, and non-contact dryers include, among others, infrared dryers and suspending air dryers. In drying cylinder groups, the web is pressed against heated cylinders by means of a wire or by means of underpressure generated inside the cylinder, whereby the web is dried due to the effect of contact heat. Infrared dryers, then, are placed in the vicinity of the web which travels guided by rolls, and the heat energy is transmitted to the web by means of radiation. In suspending air dryers, hot air is blasted towards the web, whereby the air not only has a drying but also a supporting and guiding effect on the web.
After the application of the coating, the drying of the web must be effected without touching the web because the wet web will not endure mechanical contact. Usually, both infrared and suspending dryers or combinations thereof are used for initial drying. Cylinder drying can be used first when the coat has become semi-solid and no longer adheres to the cylinders. The cylinders are used to impose the required draw on the web.
Off-machine coating apparatuses are used to treat base paper which is manufactured on a paper machine and taken to the coating stage in rolls. The apparatus comprises an unwinder, at least one coater for each side of the web, drying means arranged after the coaters, and a reeler for reeling in the coated paper. Such a coating apparatus is quite long and complex. Particularly the length of the apparatus is a disadvantage because the increased length considerably increases the space required by the machine in a factory. Most of the length of the arrangement is taken up by drying units and guide rolls needed for guiding the web and for redirecting it.
Because a coated web can only be supported on the uncoated side of the web prior to the solidification of the coating, it is necessary to bring the web through the non-contact dryers after coating supported only on one of its sides. Therefore the web must, after it has left the coater, be formed into an open loop, the coater backing roll remaining inside the loop, so that the web can be stretched and supported in the dryer area into a loop formed by the backing roll and the guide rolls. At the other end of the loop either a group of drawing-in rolls or a creasing roll is provided which turns the web support to the coated side. The web must also be brought to the coater aligned in a certain direction because modern coaters are designed such that coat metering usually takes place below the center line of the backing roll. As a result of this, the web has to travel in a double loop at least after the second coater because the web must be guided to the dryers such that it turns into its incoming direction, and after the dryers the web must then be turned back into the travel direction of the coater apparatus. If the drawing-in group of the apparatus comprises a group of drying cylinders, it is usually necessary to take the web via the group of drying cylinders after one of the coating and drying stages, whereby it travels backwards on the cylinders in the direction of the coating apparatus. As a result, a great number of rolls is needed to support the web and the entire apparatus is very long because bringing the web via the non-contacting dryers and the group of drying cylinders requires a lot of space in the long direction of the apparatus.
German patent document describes a coating apparatus where an arrangement of the dryers has been aimed at allowing the same manner of drying on both sides of the web. A reduced length of the apparatus has been sought by arranging the web to travel in loops on top of one another whereby a considerable number of guide rolls is required for guiding the web, particularly over the section for drying the second side of the web. The length of the apparatus is not essentially reduced compared to other coaters; instead the complex travel of the web can only be used to reduce the increase in the length of the apparatus which is due to the complex drying section.
In the apparatus disclosed in PCT Application No. WO 95/30049, the travel of the web is slightly simplified after the second coater by means of an air-turning device. In this solution, the web is turned onto its coated side prior to drying round the air-turning device, whereby, after the turning gear, the web can be guided directly to the non-contacting dryer, and a straighter travel of the web is achieved. However, the length of this apparatus is also considerable, and for web guiding, several guide rolls are required. Even here, the structure of the film transfer coaters determines the incoming angle as well as the angle of departure of the web, and the travel of the web must be arranged accordingly.
European Patent Application No. 93112695 describes a coating apparatus wherein the coat is applied onto the web surface by means of a film transfer roll. The film transfer roll and its backing roll are arranged on top of one another or essentially on top of one another and the web departs in a horizontal or an essentially horizontal position. After the coaters the web is guided by means of one or several air beams such that its direction is altered. In practice, the web is turned downwards at a low-gradient angle. After turning, the web is dried using a non-contacting dryer and drying cylinders. This solution achieves reasonably good runnability because after coating the web is guided non-contactingly, but if several non-contacting dryers are required for obtaining sufficient drying capacity, a very long machine results. The air beams can only be used to provide very gentle turning, so that the dryers after the turning must be positioned essentially in the direction of the web. Thus, in practice, the apparatus must be constructed at one level only, and the web can not be guided to travel in elevation in order to shorten the structure. Hence, the aim of the present invention is to create certain kind of conditions for drying after the film transfer coater.